Distillation separation apparatus and method of cleaning a reboiler/condenser of the apparatus

ABSTRACT

A distillation apparatus comprises a reboiler/condenser ( 30 ), cleanable in operation, being duplicated by another reboiler/condenser ( 40 ).  
     The apparatus comprises neither means for stopping gas to be condensed from being sent to the condenser to be cleaned nor means for stopping vaporized liquid from being withdrawn from the condenser to be cleaned.

DISTILLATION SEPARATION APPARATUS AND METHOD OF CLEANING A REBOILER/CONDENSER OF THE APPRATUS

[0001] The present invention relates to a distillation separation apparatus, in particular a cryogenic distillation separation apparatus and to a method of cleaning a reboiler/condenser of the apparatus.

[0002] To achieve good energy performance in an apparatus for separating air cryogenically requires the use of a heat exchanger making it possible to provide reboiling for the low-pressure column and reflux for the medium-pressure column.

[0003] Two fluids are used:

[0004] medium-pressure gaseous nitrogen that has to be liquefied;

[0005] low-pressure liquid oxygen that has to be vaporized.

[0006] The presence in air of impurities such as CO₂, H₂O, NO_(x) or hydrocarbons requires fitting pre-treatment units making it possible to obtain a fluid compatible with the cryogenic distillation processes.

[0007] However, because of operating problems (incorrect operation, control errors, etc.) or because the air to be treated is of abnormal quality, small amounts of impurities may pass through and get into the distillation columns and, more precisely, into the core of the liquid oxygen vaporizer.

[0008] Many methods exist for solving this problem: purging, LO filters, etc., but all have the drawback of being imperfect.

[0009] The solution normally employed consists in stopping the apparatus and “defrosting” it so as to vaporize these residual impurities.

[0010] The evolution in the downstream technologies allowing the downstream gas consumer to operate his plants for longer and longer periods between shutdowns naturally forces the air separation units to also operate for long times without shutting down. The practice of shutting down for cleaning the reboiler every 2 to 3 years must therefore disappear in favour of solutions allowing the plants to operate continuously for 5 to 6 years, or even to be able to clean the reboiler without interrupting the production.

[0011] The concept consists in fitting the reboilers on the outside of the columns and in providing partial isolation by modules.

[0012] It is known from DE-A-1501760 and from J03-152383 to use an air separation apparatus comprising a medium-pressure column, a low-pressure column and two reboiler/condensers outside the columns, each being fed with nitrogen from the medium-pressure column and with liquid oxygen from the low-pressure column.

[0013] One object of the invention is to provide a distillation apparatus comprising at least one column having at least two bath reboiler/condensers located outside the column, means for sending a gas enriched with a more volatile component from the column to each of the reboiler/condensers through at least a first respective pipe, means for sending a liquid enriched with a more volatile component from each of the reboiler/condensers to the column through at least a second respective pipe, means for sending liquid enriched with a less volatile component to each of the reboiler/condensers through at least a third respective pipe, means for withdrawing a gas enriched with a less volatile component from each of the reboiler/condensers through at least a fourth respective pipe, means for sending a mixture comprising at least the more volatile component and the less volatile component to the column and means for withdrawing at least one fluid enriched with one of the components from the column, characterized in that, in order to allow one of the reboiler/condensers to be cleaned, the apparatus comprises means for stopping liquid enriched with the more volatile component from being sent to the column of the reboiler/condenser to be cleaned and means for stopping the liquid enriched with the less volatile component from being sent to the reboiler/condenser to be cleaned and does not comprise means for stopping gas enriched with the less volatile component from being withdrawn from the reboiler/condenser to be cleaned and/or means for stopping gas enriched with the more volatile component from being sent from the column to the reboiler/condenser to be cleaned.

[0014] Another object of the invention is to provide an air separation apparatus comprising a medium-pressure column, a low-pressure column, at least two bath reboiler/condensers located outside the columns, means for sending a gas enriched with nitrogen from the medium-pressure column to each of the reboiler/condensers through at least a first respective pipe, means for sending a liquid enriched with nitrogen from each of the reboiler/condensers to the medium-pressure column through at least a second respective pipe, means for sending liquid enriched with oxygen from the low-pressure column to each of the reboiler/condensers through at least a third respective pipe, means for sending a gas enriched with oxygen from each of the reboiler/condensers to the low-pressure column through at least a fourth respective pipe, means for sending air to the medium-pressure column and means for withdrawing at least one fluid from one of the columns as product and means for sending a liquid enriched with oxygen from the medium-pressure column to the low-pressure column, characterized in that, in order to allow one of the reboiler/condensers to be cleaned, the apparatus comprises means for stopping liquid enriched with nitrogen from being sent to the medium-pressure column from the reboiler/condenser to be cleaned and means for stopping liquid enriched with oxygen from being sent from the low-pressure column to the reboiler/condenser to be cleaned and does not comprise means for stopping gas enriched with oxygen from being sent to the low-pressure column from the reboiler/condenser to be cleaned and/or means for stopping gas enriched with nitrogen from being sent from the medium-pressure column to the reboiler/condenser to be cleaned.

[0015] Another object of the invention is to provide an air separation apparatus comprising a column, at least two bath reboiler/condensers located outside the column, means for sending a gas enriched with nitrogen from the column to each of the reboiler/condensers through at least a first respective pipe, means for sending a liquid enriched with nitrogen from each of the reboiler/condensers to the column through at least a second respective pipe, means for sending liquid enriched with oxygen from the column to each of the reboiler/condensers through at least a third respective pipe, means for sending a gas enriched with oxygen from each of the reboiler/condensers into at least a fourth respective pipe, means for sending air to the column and means for withdrawing at least one fluid from the column as product, characterized in that, in order to allow one of the reboiler/condensers to be cleaned, the apparatus comprises means for stopping liquid enriched with nitrogen from being sent to the column from the reboiler/condenser to be cleaned and means for stopping liquid enriched with oxygen from being sent from the column to the reboiler/condenser to be cleaned and does not comprise means for stopping gas enriched with oxygen from being sent into the fourth pipe from the reboiler/condenser to be cleaned and/or means for stopping gas enriched with nitrogen from being sent from the medium-pressure column to the reboiler/condenser to be cleaned.

[0016] Another object of the invention is to provide an air separation apparatus, comprising a medium-pressure column, a low-pressure column, at least two bath reboiler/condensers located outside the columns, means for sending a gas enriched with nitrogen from the medium-pressure column to a reboiler/condenser of the low-pressure column, means for sending air to each of the reboiler/condensers through at least a first respective pipe, means for sending at least partially condensed air from each of the reboiler/condensers to the medium-pressure column through at least a second respective pipe, means for sending liquid enriched with oxygen from the low-pressure column to each of the reboiler/condensers through at least a third respective pipe, means for sending a gas enriched with oxygen from each of the reboiler/condensers to the low-pressure column through at least a fourth respective pipe, means for sending air to the medium-pressure column and means for withdrawing at least one fluid from one of the columns as product and means for sending a liquid enriched with oxygen from the medium-pressure column to the low-pressure column,

[0017] characterized in that, in order to allow one of the reboiler/condensers to be cleaned, the apparatus comprises means for stopping at least partially condensed air from being sent to the medium-pressure column from the reboiler/condenser to be cleaned and means for stopping liquid enriched with oxygen from being sent from the low-pressure column to the reboiler/condenser to be cleaned and does not comprise means for stopping gas enriched with oxygen from being sent to the low-pressure column from the reboiler/condenser to be cleaned and/or means for stopping air from being sent to the reboiler/condenser to be cleaned.

[0018] According to other optional aspects, the apparatus comprises:

[0019] means for sending a nitrogen-enriched purge gas into the second pipe of the reboiler/condenser to be cleaned and an oxygen-enriched purge gas into the third pipe of the reboiler/condenser to be cleaned;

[0020] venting means connected to the first and fourth pipes of the reboiler/condenser to be cleaned.

[0021] The venting means and the means for sending a purge gas may comprise calibrated orifices, automatic control valves or any other means allowing the flow rates to be controlled.

[0022] Preferably, the rates of venting the first and fourth pipes are respectively greater than the rates of purging the second and third pipes.

[0023] The apparatus comprises a purge pipe connected to the bottom of the bath or to a low point on the third pipe of each reboiler/condenser.

[0024] The apparatus may comprise a purge pipe connected to the second pipe from the bath of each reboiler/condenser.

[0025] Another object of the invention is to provide a method for cleaning a reboiler/condenser of an apparatus according to one of the preceding claims, comprising the steps of:

[0026] i) isolating the valves of the second and third pipes;

[0027] ii) purging the oxygen-enriched liquid of the reboiler/condenser to be cleaned through a purge pipe;

[0028] iii) opening the vents of the first and/or fourth pipes and/or;

[0029] iv) sending nitrogen-enriched flushing gas into the second pipe; and/or

[0030] v) sending oxygen-enriched flushing gas into the third pipe.

[0031] Preferably, the oxygen-enriched flushing gas has an oxygen content preferably greater than or equal to that of the liquid in the bath of the reboiler/condenser in the operating state and the nitrogen-enriched purge gas has a nitrogen content preferably greater than or equal to that of the nitrogen-enriched gas sent to the reboiler/condenser in the operating state, in such a way as not to appreciably contaminate the production parts of the apparatus. Optionally, the oxygen-enriched flushing gas has an oxygen content at least equal to that of the liquid in the bath of the reboiler/condenser in the operating state.

[0032] Optionally, the nitrogen-enriched flushing gas has a nitrogen content at least equal to that of the nitrogen-enriched gas sent to the reboiler/condenser in the operating state.

[0033] Preferably, the oxygen-enriched or nitrogen-enriched flushing gas undergoes a critical expansion before injection into the reboiler/condenser to be cleaned.

[0034] The flushing gases may be at temperatures above the sublimation or vaporization temperature of the impurities present in the reboiler/condenser.

[0035] Optionally, the liquid enriched with the more volatile component contained in the reboiler/condenser to be cleaned is purged after step i).

[0036] Preferably, the oxygen-enriched flushing gas has an oxygen content at least equal to that of the liquid in the bath of the reboiler/condenser in the operating state.

[0037] According to optional aspects of the invention:

[0038] the nitrogen-enriched flushing gas has a nitrogen content at least equal to that of the nitrogen-enriched gas sent to the reboiler/condenser in the operating state;

[0039] the oxygen-enriched or nitrogen-enriched flushing gas undergoes a critical expansion before injection into the reboiler/condenser to be cleaned;

[0040] the flushing gases are at temperatures above the sublimation or vaporization temperature of the impurities present in the reboiler/condenser;

[0041] the liquid enriched with the more volatile component contained in the reboiler/condenser to be cleaned is purged after step i);

[0042] the rates of venting the first and fourth pipes are respectively greater than the rates of purging the second and third pipes.

[0043] The first and fourth pipes are not closed during the cleaning.

[0044] Production remains possible during the periodic regeneration of an isolated module by defrosting, but with lower energy efficiency.

[0045] This method is applicable without shutting down the production and without fitting expensive large-diameter cryogenic valves (in the nitrogen and oxygen gas circuits).

[0046] The method is summarized as follows:

[0047] isolate the liquid lines sending the oxygen-enriched liquid to the reboiler and discharging the condensed liquid (air or nitrogen) from the reboiler;

[0048] purge at least the oxygen-enriched liquid;

[0049] open the vents (the amounts of gas lost are small);

[0050] flush with nitrogen;

[0051] flush with oxygen.

[0052] Suitably dimensioning the calibrated orifices (FO) makes it possible to eliminate any risk of excessive depressurization or pressurization of the columns.

[0053] The invention will now be described in greater detail with reference to the figures which are schematic drawings of apparatuses according to the invention.

[0054] The apparatus of FIG. 1 is a double column comprising a medium-pressure column 10 and a low-pressure column 20. A stream of air 11 is sent to the medium-pressure column where it separates into a nitrogen-enriched gas at the top of the column and an oxygen-enriched liquid at the bottom of the column. The oxygen-enriched liquid 13 is sent to an intermediate point on the low-pressure column.

[0055] The apparatus comprises two reboiler/condensers 30, 40, only one of which is operating at the same time.

[0056] Thus, in normal operation, the reboiler/condenser 40 receives nitrogen-enriched gas via the first pipe 1 and oxygen-rich liquid via the third pipe 3. The oxygen vaporizes to form gaseous oxygen which leaves through the fourth pipe 4 and the nitrogen-enriched liquid thus formed leaves the reboiler/condenser 30 via the second pipe 2 in order to be returned to the medium-pressure column and possibly to the low-pressure column.

[0057] In this case, the liquid-oxygen inlet valves 23 of the reboiler/condenser 30 remain closed, and therefore all the vaporization takes place in the reboiler 40.

[0058] When it is necessary to clean the latter, the liquid-oxygen inlet valves 23 in the third pipe 3 of the reboiler 40 are opened, the latter assuming the role of reboiler for the double column. In this way, it is the pipes 2 and 3 of the reboiler 40 which deliver the liquid nitrogen and vaporized oxygen to the apparatus.

[0059] In order to clean the reboiler 30, the gaseous nitrogen continues to be allowed to enter the latter via the pipe 1. However, the inlet for liquid oxygen through the pipe 3 is closed by the valve 23 and the liquid oxygen remaining in the reboiler is purged via the purge line 7 from the bottom of the bath or connected to the pipe itself (not illustrated). Optionally, in order to minimize the risk of thermal shock, the liquid nitrogen remaining in the reboiler is purged via a purge line 9 of the pipe 2, the valve 22 of the pipe 2 being closed.

[0060] Next, vents (V) 16, 18 in the pipes 1 and 4 are opened so that the gas streams leave the reboiler/condenser 30 through the calibrated orifices FO.

[0061] The next step consists in sending an oxygen-rich flushing gas into the third pipe 3 downstream of the valve 23 in order to flush out the impurities from the oxygen passages of the reboiler 30. This gas is generally at ambient temperature and is at least as pure as the oxygen vaporized by the reboiler in normal use.

[0062] A nitrogen-rich flushing gas is sent into the second pipe upstream of the valve 22. This gas serves to flush out the impurities from the nitrogen passages.

[0063] In both cases the flushing gases must be at temperatures above the sublimation/vaporization temperature of the impurities (CO₂, H₂O, NO_(x), C_(n)H_(m), etc.) present in the bath.

[0064] After having been used to remove the remaining impurities, the flushing gases are sent to the respective vents 6, 8. This necessarily leads to gas losses, but these prove to be acceptable however.

[0065] For example, for a 2000 ton/day air separation apparatus, the maximum instantaneous loss of nitrogen from the medium-pressure column is 1% of the incoming nitrogen and the maximum instantaneous loss of oxygen from the low-pressure column is 5% of the oxygen production.

[0066] The flushing gases may come from external storage tanks.

[0067] In the case of FIG. 2, a single column 60 has two top reboiler/condensers 30, 40.

[0068] In this case, it is the rich liquid from the bottom of the column which serves to vaporize the overhead nitrogen in the reboilers 30, 40.

[0069] In the case of FIG. 3, a double column comprises two reboiler/condensers 30, 40 for vaporizing the oxygen from the bottom of the low-pressure column by heat exchange with a stream of air.

[0070] The air partially condensed in the reboiler 30 feeds the medium-pressure column.

[0071] The overhead nitrogen of the medium-pressure column vaporizes in a third reboiler 70 inside the column.

[0072] Obviously, each of the first, second, third and fourth pipes may comprise a plurality of pipes.

[0073] The column having two top bath reboilers may be an argon production column, a low-pressure column, a medium-pressure column, an intermediate-pressure column of a triple column, or a mixing column of an air separation apparatus.

[0074] The column can separate a gas mixture from air or a mixture comprising at least 1% hydrogen and/or at least 1% carbon monoxide and/or at least 1% nitrogen and/or 1% methane. 

1. Distillation apparatus comprising at least one column (10, 60) having at least two bath reboiler/condensers (30, 40) located outside the column, means (1) for sending a gas enriched with a more volatile component from the column to each of the reboiler/condensers through at least a first respective pipe, means (2) for sending a liquid enriched with a more volatile component from each of the reboiler/condensers to the column through at least a second respective pipe, means (3) for sending liquid enriched with a less volatile component to each of the reboiler/condensers through at least a third respective pipe, means (4) for withdrawing a gas enriched with a less volatile component from each of the reboiler/condensers through at least a fourth respective pipe, means (11) for sending a mixture comprising at least the more volatile component and the less volatile component to the column and means for withdrawing at least one fluid enriched with one of the components from the column, characterized in that, in order to allow one (30) of the reboiler/condensers to be cleaned, the apparatus comprises means (22) for stopping liquid enriched with the more volatile component from being sent to the column of the reboiler/condenser to be cleaned and means (23) for stopping the liquid enriched with the less volatile component from being sent to the reboiler/condenser to be cleaned and does not comprise means for stopping gas enriched with the less volatile component from being withdrawn from the reboiler/condenser to be cleaned and/or means for stopping gas enriched with the more volatile component from being sent from the column to the reboiler/condenser to be cleaned.
 2. Air separation apparatus according to claim 1 comprising a medium-pressure column (10), a low-pressure column (20), at least two bath reboiler/condensers (30, 40) located outside the columns, means (1) for sending a gas enriched with nitrogen from the medium-pressure column to each of the reboiler/condensers through at least a first respective pipe, means (2) for sending a liquid enriched with nitrogen from each of the reboiler/condensers to the medium-pressure column through at least a second respective pipe, means (3) for sending liquid enriched with oxygen from the low-pressure column to each of the reboiler/condensers through at least a third respective pipe, means (4) for sending a gas enriched with oxygen from each of the reboiler/condensers to the low-pressure column through at least a fourth respective pipe, means (11) for sending air to the medium-pressure column and means for withdrawing at least one fluid from one of the columns as product and means (13) for sending a liquid enriched with oxygen from the medium-pressure column to the low-pressure column, characterized in that, in order to allow one of the reboiler/condensers to be cleaned, the apparatus comprises means (22) for stopping liquid enriched with nitrogen from being sent to the medium-pressure column from the reboiler/condenser to be cleaned and means (23) for stopping liquid enriched with oxygen from being sent from the low-pressure column to the reboiler/condenser to be cleaned and does not comprise means for stopping gas enriched with oxygen from being sent to the low-pressure column from the reboiler/condenser to be cleaned and/or means for stopping gas enriched with nitrogen from being sent from the medium-pressure column to the reboiler/condenser to be cleaned.
 3. Air separation apparatus according to claim 1, comprising a column (60), at least two bath reboiler/condensers (30, 40) located outside the column, means (1) for sending a gas enriched with nitrogen from the column to each of the reboiler/condensers through at least a first respective pipe, means (2) for sending a liquid enriched with nitrogen from each of the reboiler/condensers to the column through at least a second respective pipe, means (3) for sending liquid enriched with oxygen from the column to each of the reboiler/condensers through at least a third respective pipe, means (4) for sending a gas enriched with oxygen from each of the reboiler/condensers into at least a fourth respective pipe, means (11) for sending air to the column and means for withdrawing at least one fluid from the column as product, characterized in that, in order to allow one of the reboiler/condensers to be cleaned, the apparatus comprises means (22) for stopping liquid enriched with nitrogen from being sent to the column from the reboiler/condenser to be cleaned and means (23) for stopping liquid enriched with oxygen from being sent from the column to the reboiler/condenser to be cleaned and does not comprise means for stopping gas enriched with oxygen from being sent into the fourth pipe from the reboiler/condenser to be cleaned and/or means for stopping gas enriched with nitrogen from being sent from the medium-pressure column to the reboiler/condenser to be cleaned.
 4. Air separation apparatus according to claim 1, comprising a medium-pressure column (10), a low-pressure column (20), at least two bath reboiler/condensers (30, 40) located outside the columns, means for sending a gas enriched with nitrogen from the medium-pressure column to a reboiler/condenser (70) of the low-pressure column, means (1) for sending air to each of the reboiler/condensers through at least a first respective pipe, means (2) for sending at least partially condensed air from each of the reboiler/condensers to the medium-pressure column through at least a second respective pipe, means (3) for sending liquid enriched with oxygen from the low-pressure column to each of the reboiler/condensers through at least a third respective pipe, means (4) for sending a gas enriched with oxygen from each of the reboiler/condensers to the low-pressure column through at least a fourth respective pipe, means for sending air (11) to the medium-pressure column and means for withdrawing at least one fluid from one of the columns as product and means (13) for sending a liquid enriched with oxygen from the medium-pressure column to the low-pressure column, characterized in that, in order to allow one of the reboiler/condensers to be cleaned, the apparatus comprises means (22) for stopping at least partially condensed air from being sent to the medium-pressure column from the reboiler/condenser to be cleaned and means (23) for stopping liquid enriched with oxygen from being sent from the low-pressure column to the reboiler/condenser to be cleaned and does not comprise means for stopping gas enriched with oxygen from being sent to the low-pressure column from the reboiler/condenser to be cleaned and/or means for stopping air from being sent to the reboiler/condenser to be cleaned.
 5. Apparatus according to one of claims 2, 3 and 4, comprising means (15, 17) for sending a purge gas enriched with nitrogen, possibly air, into the second pipe of the reboiler/condenser to be cleaned and/or a purge gas enriched with oxygen into the third pipe of the reboiler/condenser to be cleaned.
 6. Apparatus according to one of claims 2, 3 and 4, comprising venting means (16, 18) connected to the first and fourth pipes of the reboiler/condenser to be cleaned.
 7. Apparatus according to claim 5 or 6, in which the venting means and/or the means for sending purge gas comprise calibrated orifices.
 8. Apparatus according to claim 6 or 7, in which the venting means and/or the means for sending purge gas comprise automatic control valves.
 9. Apparatus according to one of the preceding claims, comprising an oxygen-enriched liquid purge pipe (7) connected to the bottom of the bath of each reboiler/condenser or to the third pipe.
 10. Apparatus according to one of the preceding claims, comprising a nitrogen-enriched liquid purge pipe (9) connected to the second pipe (2).
 11. Method for cleaning a reboiler/condenser of an apparatus according to one of the preceding claims, comprising the steps of: i) isolating the second and third pipes of the reboiler/condenser to be cleaned by closing the access to the column(s); ii) purging the liquid enriched with the less volatile component of the reboiler/condenser to be cleaned through a purge pipe; iii) opening the vents of the first and/or fourth pipes and/or; iv) sending nitrogen-enriched flushing gas into the second pipe; and/or v) sending oxygen-enriched flushing gas into the third pipe.
 12. Method according to claim 11, in which the oxygen-enriched flushing gas has an oxygen content at least equal to that of the liquid in the bath of the reboiler/condenser in the operating state.
 13. Method according to claim 11 or 12, in which the nitrogen-enriched flushing gas has a nitrogen content at least equal to that of the nitrogen-enriched gas sent to the reboiler/condenser in the operating state.
 14. Method according to one of claims 11, 12 and 13, in which the oxygen-enriched or nitrogen-enriched flushing gas undergoes a critical expansion before injection into the reboiler/condenser to be cleaned.
 15. Method according to claims 11 to 14, in which the flushing gases are at temperatures above the sublimation or vaporization temperature of the impurities present in the reboiler/condenser.
 16. Method according to claims 11 to 15, in which the liquid enriched with the more volatile component contained in the reboiler/condenser to be cleaned is purged after step i).
 17. Method according to claims 11 to 16, in which the rates of venting the first and fourth pipes are respectively greater than the rates of purging the second and third pipes. 